Convert PPFD to DLI

What is DLI?

DLI stands for daily light integral. It says how many photosynthetically usable light particles land on a square metre over the course of a day — given in mol/m²/d.

PPFD, by contrast, is a snapshot: how much light is arriving right now, in µmol/m²/s. But what counts for the plant is not the moment, it is the daily ration. A weak lamp running long can deliver more light to the same plant than a strong one running briefly.

That is exactly why DLI is the more honest figure. Knowing only your PPFD, you still do not know whether your plants are getting enough — the photoperiod is missing from the sum.

The formula — and the rule of thumb behind it

The conversion is plain arithmetic: DLI = PPFD × hours × 3600 ÷ 1,000,000. The 3600 turns seconds into hours, the million turns micromoles into moles. If you want it in your head, fold both together: PPFD × hours × 0.0036.

Example: 600 µmol/m²/s over 18 hours gives 600 × 18 × 0.0036 = 38.88 mol/m²/d. The same lamp on a 12-hour flowering schedule reaches only 25.92 — a third less, without anything changing on the fixture.

It works backwards just as well: PPFD = DLI × 1,000,000 ÷ (hours × 3600). Aiming for 40 mol/m²/d over 18 hours means you need 617 µmol/m²/s. That direction is the more useful one in practice — you know your target and are looking for the setting, not the other way round.

How much DLI does cannabis need?

The reference values in the table above cover ordinary indoor growing: seedlings and clones 10–20, vegetative 20–40, flower 30–45 mol/m²/d. They rise with the plant — a seedling with two leaves cannot process an amount of light that a mature plant turns into yield.

And here is what most tables leave out: every DLI value belongs to a photoperiod. The 45 in flower mean 1,042 µmol/m²/s at 12 hours of light — the absolute upper end for a tent without CO₂. At 14 hours the same 45 mol are only 893 µmol/m²/s, an entirely ordinary figure. Same daily total, completely different demand on the fixture.

In practice: if you want the upper half of the flowering range, duration usually gets you there more cheaply than intensity. More hours cost electricity one for one; more intensity additionally costs cooling, distance and eventually yield.

Why DLI tables online contradict each other

Compare a few grow sites and you will find figures for flower ranging from 30 to 65 mol/m²/d. That is not because plants differ, but because of two mistakes that keep circulating.

First, the CO₂ figures. Numbers above roughly 50 almost always come from commercial rooms running 1,000 to 2,000 ppm carbon dioxide. Without enrichment the plant cannot use that much light — it saturates considerably earlier, and every further watt only lands on the electricity bill.

Second, the conversion itself. It happens that a page puts PPFD recommendations and DLI recommendations side by side that do not add up — say "600 to 900 µmol/m²/s at 12 hours" and in the same breath "DLI 35 to 50". Run the numbers and 900 µmol/m²/s over 12 hours gives 38.88 mol/m²/d, not 50. The upper bound is simply derived wrongly.

That is why the table on this page carries the matching PPFD and the number of hours next to every DLI value — and both come out of the same formula as the calculator above. You can check every row in two clicks.

How many hours of light?

The photoperiod is the second input — and unlike intensity it costs nothing but electricity. Common practice is 18 hours in propagation, 18 hours in veg (some run 20 or even 24) and 12 hours in flower.

Those 12 hours in flower are not a yield decision but a biological one: photoperiod strains only flower once the dark period is long enough. So that number is fixed — if you want more light in flower, you have to go through intensity, not duration. Which is exactly why flower is the stage where the fixture reaches its limit.

With autoflowers that does not apply. They flower on a timer and usually run 18 or 20 hours throughout, which lets them reach high daily totals at comparatively relaxed PPFD figures. Set the calculator to 20 hours and you see the difference immediately.

Shortened cycles such as 11/13 or 10/14, which some run in late flower, are a special case. They lower the daily total noticeably: going from 12 to 11 hours costs roughly eight percent of DLI on the same fixture. If you try that, work the value out rather than guessing it.

Where the ceiling is

DLI is not open-ended upwards. Without CO₂ enrichment cannabis hits the limit of what it can turn into yield somewhere around 45 mol/m²/d. Anything beyond that requires added CO₂ — and even then it stops at about 55.

The reason lies in photosynthesis: light is only one of three factors. If carbon dioxide is short or the temperature is off, extra light does nothing more. At very high intensity the plant even partly closes its stomata to protect itself — yield stops rising and the leaves bleach. Whether your climate is actually in range is what the VPD calculator tells you.

Downwards the limit is softer but just as real: below roughly 10 mol/m²/d the plant stretches for the light, throwing long internodes and thin stems. That cannot be corrected later.

Measuring DLI and keeping an eye on it

DLI is not measured directly — you measure PPFD and convert. The same rules apply as for any light measurement: at canopy height, sensor level, several points spread across the footprint. Edge fall-off is considerable on almost every fixture, and a single reading from the centre suggests an evenness that is not there.

If all you have is a lux meter or a phone app, you need the detour via light type — that is what the lux to PPFD calculator is for. The result is an estimate, but usually good enough to place a grow in the right stage.

One practical note: DLI changes with every adjustment — lamp raised, dimmed, timer altered. Noting the value together with the date lets you see later which lighting plan actually produced the better yield. That is exactly what the grow journal in CRIS is for.